* lib/versioner: Reduce surface area
This is a refactor while I was anyway rooting around in the versioner.
Instead of exporting every possible implementation and the factory and
letting the caller do whatever, this now encapsulates all that and
exposes a New() that takes a config.VersioningConfiguration.
Given that and that we don't know (from the outside) how a versioner
works or what state it keeps, we now just construct it once per folder
and keep it around. Previously it was recreated for each restore
request.
* unparam
* wip
Also fixes an issue where the discovery cache call would only return the
newest cache entry for a given device instead of the merged addresses
from all cache entries (which is more useful).
GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/3344
